Other than self gratification, why would one want to become a Master diver? What benefits or what does it mean to hold a Master Certification? (other than to myself)

The NAUI master scuba diver course is separate from the "dive master".

It strengthens and refreshes your skills in rescue diving, deep diving, night diving, search and recovery, and many more.
It also goes into depth on equipment, physics of diving, environment, and rescue.